1. Field of the Invention
The present invention relates to a numerically controlled lathe having an annular multiple-tool head, a turret head and a rear machining head. More particularly, the present invention pertains to a numerically controlled lathe having an annular multiple-tool head having a plurality of turning tools disposed in opposed relation to each other in the directions X and Y, a turret head capable of differentially rotating a cutting tool, for example, a drill, a tap, etc., relative to the main spindle to effect machining at a cutting speed suitable for the cutting tool simultaneously with the machining effected by the multiple-tool head, and a rear machining head which mainly effects machining of a workpiece held on a rear machining spindle provided on the turret head.
2. Description of the Prior Art
A typical conventional NC lathe has a turret head. The turret head has a plurality of tools and indexes a tool to be used in accordance with the kind of machining to be effected, thereby carrying out a desired machining operation. There are known various kinds of turret head structure which have different tool indexing mechanisms, for example, the rotational indexing turret head and the linear indexing turret head. Among them is Japanese Patent Laid-Open (KOKAI) No. 61-51203 (1986) wherein a multiplicity of tools are disposed so as to face each other across a workpiece in the direction of the Y-axis. This type of the turret head has advantages, for example, a short indexing time, that is, the time needed to change tools is short. To effect rear machining, i.e., machining of the rear of a workpiece held by a chuck, with the conventional NC lathe described above, it is common practice to suspend the main spindle, remove the workpiece from the chuck, reverse the workpiece and re-chuck the reversed workpiece before effecting rear machining. The reversing of the workpiece is carried out by a manual operation or using a robot hand.
Among various kinds of conventional NC lathe, there is known an opposed twin spindle NC lathe wherein a secondary-spindle is provided in opposed relation to a main spindle. This type of NC lathe also has two turret heads in order to machine a workpiece held on each spindle. Each spindle is provided with a work chuck, and the secondary-spindle is numerically controlled to chuck the workpiece delivered from the main spindle side.
In relation to the opposed twin spindle NC lathe, a machining apparatus and a machining method wherein, after one end of a workpiece has been machined on the main spindle, the workpiece is delivered to the chuck on the secondary-spindle opposing the main spindle to machine the other end of the workpiece are known [for example, Japanese Patent Laid-Open (KOKAI) No. 63-272401 (1988) and Japanese Patent Post-Exam Publication No. 51-14186 (1976)]. In the above-described opposed twin spindle NC lathe, however, when a workpiece is to be subjected to rear machining, the workpiece is delivered in such a manner that two ends thereof are respectively held with a chuck provided on the main spindle and a chuck provided on the secondary-spindle. Japanese Utility Model Post-Exam Publication No. 53-33262 (1978) discloses an apparatus wherein a work chuck and a rear machining head are provided on a turret head. Since this work chuck has no servomotor therefor, it must be externally driven.
Therefore, each of the main spindle and the secondary-spindle has a servomotor for movement along the Z-axis, that is, the axis of the main spindle. The headstock must be driven together with the head. This arrangement merely features that the secondary-spindle has the ordinary main spindle arrangement that the spindle effects delivery of a workpiece and rotates while holding it.